My 77 now only blows out of the center vents. I took the dash apart replaced all vacuum lines and checked cables. How do I continue to trouble shoot what is probably a vacuum / solinoid /diaphragm (bottom of blower motor) problem?

I actually think it's simpler than that. Are your side vent tubes connected at both ends? I only ask because when I got my 78, they were both cracked an the heat never made it to the vents.

Replaced all of the tubes which were dry rotted when I got the car. I tink my multimeter and I will have a busy day tomorrow. Thanks Steve

Have you checked the mode door actuator? It's on the left side of the heater box. Two vacuum hoses. One to open and one to close.

Are those the 2 hoses that run up under the dash on the left hooked up to a diaphragm of some sort?I did replace those lines but have not checked the actuator if that is what the diaphragm is.I also checked the cable bottom left that works the lower doors which are ok. I guess a vacuume pump is in order.